High‐Rate Organic Cathode Constructed by Iron‐Hexaazatrinaphthalene Tricarboxylic Acid Coordination Polymer for Li‐Ion Batteries

Abstract The sluggish ion‐transport in electrodes and low utilization of active materials are critical limitations of organic cathodes, which lead to the slow reaction dynamics and low specific capacity. In this study, the hierarchical tube is constructed by iron‐hexaazatrinaphthalene tricarboxylic acid coordination polymer (Fe‐HATNTA), using HATNTA as the self‐engaged template to coordinate with Fe2+ ions. This Fe‐HATNTA tube with hierarchical porous structure ensures the sufficient contact between electrolyte and active materials, shortens the diffusion distance, and provides more favorable transport pathways for ions. When employed as the cathode for rechargeable Li‐ion batteries, Fe‐HATNTA delivers a high specific capacity (244 mAh g−1 at 50 mA g−1, 91% of theoretical capacity), excellent rate capability (128 mAh g−1 at 9 A g−1), and a long‐term cycle life (73.9% retention over 3000 cycles at 5 A g−1). Moreover, the Li+ ions storage and conduction mechanisms are further disclosed by the ex situ and in situ characterizations, kinetic analyses, and theoretical calculations. This work is expected to boost further enthusiasm for developing the hierarchical structured metal‐organic coordination polymers with superb ionic storage and transport as high‐performance organic cathodes.

HRTEM images further confirm the numerous mesopores, which are constructed by accumulation of crystals, providing an alternative for Li + ion transport and shortening Li + ion diffusion distance.
s6 Figure S5. The corresponding morphologies of the obtained products recorded at different reaction stages: FESEM and TEM images of (a, e) 0h, (b, f) 12h, (c, g) 24h, The formation mechanism of the tube-like nanostructure is further revealed by investigating the growth process. The varied morphologies at different reaction stages are characterized via FESEM and TEM observations ( Figure S5). At the initial stage of the reaction, driven by a combination of the non-covalent interactions such as hydrogen bonding and π-π stacking in the solution, HATNTA molecules assemble into a fibrous structure ( Figure S5a, e). As the reaction time is further prolonged, iron ions begin to coordinate with HTANTA to form Fe-HATNTA nanocrystals epitaxially growing on the surface of HATNTA nanorod assemblies ( Figure S5b, f).
In this stage, Fe-HATNTA nanocrystals keep the growth on the surface of HATNTA nanorod-shaped assemblies, while HATNTA rod-shaped assemblies are continuously consumed ( Figure S5c, g). At the end of the reaction, the rod-shaped HATNTA reactant is completely consumed to generate hierarchical Fe-HATNTA nanotubes consisting of numerous nanoparticles ( Figure  S5d, h).
As shown in Figure S6, the signal at 3400-2700 cm -1 is contributed from the s9 Figure S8. The proposed two-step three-electron transfer mechanism in Fe-HATNTA.
The corresponding reactions of the redox peaks are in accordance with the proposed two-step three-electron transfer mechanism for HATN-based derivatives, as shown in Figure S8. Therefore, the Fe-HTATNTA exhibited two redox couples (2.42 V/2.46 and 1.84 V/2.12) in cyclic voltammetry (CV) curves.  The charge/discharge curves show the according specific capacity at current densities from 0.2 to 9 A g -1 . It can be easily seen the reversible specific capacity of 128 mAh g -1 at the high current density of 9 A g -1 , indicating the excellent high-rate permance.
s12 Figure S11. a) Rate performance of HATNTA at different current densities. b) Charge-discharge profiles of HATNTA at different current densities. Cycling performance for at a current density of 0.2 A g -1 . c-d) Rate performance at various current densities.
Additionally, this MOCPs system is rendered electrochemically brilliant with the high ration of active materials (active material: carbon black: polyvinylidene fluoride = 8:1:1).
When cycled at 200 mA g -1 , the capacity still sustains at 127.9 mAh g -1 over 200 cycles, yielding a capacity retention of 89.3% ( Figure S12b). The corresponding charge/discharge curves are highly overlapped as shown in Figure S12a. It should be mentioned that the rate capability is also striking in this high ratio of active material, demonstrating a high capacity of 112.1 mAh g -1 at a high loading current of 2 A g -1 ( Figure S12c, d). When the current density is reverted to 40 mA g -1 , the specific capacity also gradually recovers to its initial level of 161.6 mAh g -1 .
s14 Figure S13. High-resolution O 1s spectra of Fe-HATNTA at different electrochemical states.
As shown in Figure S13, the O 1s deconvolution produced the peaks centered approximately at 532.7 and 532.1, which are separately assigned to C-O and C=O, and there is no obvious change occurred in the different charge state, pointing to the noninvolvement of COOgroups for redox reaction.
s15 Figure S14. High-resolution Fe 2p spectra of Fe-HATNTA at different electrochemical states.
In addition, in the Fe 2p XPS spectra ( Figure S14 ), the deconvoluted peaks at 709.9 eV and 715.8 eV are attributed to Fe 2p 3/2 and its satellite, respectively. At the same time, the peaks displayed negligible change in cycling process, indicating that metal ions serve only as the bridge to stabilize Fe-HATNTA structure rather than participates in redox reaction.
After 20 cycles at 0.5A g -1 , there is no distinct change occurred for Fe 2p XPS spectra in Fe-HATNTA, when compared with the pristine electrode.
s17 Figure S16. The EPR spectra of Fe-HATNTA in different charge state.
As can be seen from the electron paramagnetic resonance (EPR) spectra, there is no sign of Fe 3+ ions peak in the different charge states, indicating that Fe 2+ ions didn't change into Fe 3+ during the charging-discharging process.
s18 Figure S17. Log(i) versus log(v) plots in the range of 1.7-2.5 V during cathodic sweeps. Figure S17 shows the good linear fit between log i and log v, in which b-values can be deduced from the slope of the plots.
s19 Figure S18. GITT curves of HATNTA at 50 mA g -1 and the calculated ionic diffusion coefficient.
The average of Li + ions diffusion coefficient in HATNTA is calculated to be 1.82×10 -11 cm 2 s -1 and 1.87×10 -11 cm 2 s -1 during the discharge and charge phase, respectively, which is lower than our Fe-HATNTA cathode.
s20 Figure S19. The impedance plots of HATNTA before and after 500 cycles at 5 A g -1 .
The R ct of HATNTA is as high as 370 Ω before and after cycles, indicating the poor electron transport and unfavorable electrode-electrolyte contact of HATNTA cathode.